The present invention relates to the field of integrated display device manufacture, and more specifically to an improved method and construction for the manufacture of light reflective liquid crystal display (xe2x80x9cLCDxe2x80x9d) arrays. The predominant current usage of the present inventive improved DC bias apparatus and method is in the construction of light reflective LCD arrays for projection display devices, wherein picture quality is of great importance, due to the fact that an image produced thereby will be greatly enlarged.
It is known in the art that any long term DC voltage bias in the liquid crystal of an LCD array will eventually destroy its operation. Although great effort in the design of the mirror cell and the algorithms of data presentation go into eliminating this bias, no solution has yet been found that completely eliminates the DC voltage bias.
Prior art algorithms for eliminating DC bias typically involve periodically switching the voltage on the ITO electrode between two or more voltages. For example, in one implementation of liquid crystal cell operation, the ITO has a voltage V0. An xe2x80x9conxe2x80x9d mirror has a voltage of V1 asserted thereon, while an xe2x80x9coffxe2x80x9d mirror has a voltage V0 asserted thereon. Periodically (e.g., once per frame of data), the voltage on the ITO is switched between V0 and V1, as are the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d voltages. This type of routine eliminates some, but not all, of the DC bias observed in LCD arrays.
It would be desirable to have some method or means to more completely eliminate the residual DC voltage buildup from such LCD arrays. However, to the inventor""s knowledge, no such method or means has existed in the prior art.
Accordingly, it is an object of the present invention to provide a method and apparatus for assisting in preventing the buildup of a residual DC bias in a liquid crystal display array.
It is another object of the present invention to prolong the life and improve the performance of a reflective LCD array.
It is still another object of the present invention to provide a method and apparatus for preventing the buildup of a residual DC voltage between an ITO layer of an LCD array and metal underlying the mirrors thereof.
It is yet another object of the present invention to provide a method and apparatus to prevent the migration of ions within an LCD array.
While prior art solutions have generally assumed that all electric field lines originating on the indium tin oxide (xe2x80x9cITOxe2x80x9d) layer above the liquid crystal, which go through the liquid crystal, terminate on the mirrors, the inventor has found that a small portion of electric field lines pass through the gap in the mirrors and terminate on the metal layers beneath them. If that metal has a voltage bias with respect to the ITO, then there is a residual DC bias in the liquid crystal.
Briefly, the present invention provides traces within the spaces between the mirrors of a reflective LCD array which, on average over a limited area, carry voltages which tend to prevent the migration of ions within the liquid crystal. As stated above, in one implementation of liquid crystal cell operation, the ITO is periodically switched to eliminate DC voltage bias in the liquid crystal which might be built up between the mirrors and the ITO. However, this procedure does not eliminate bias which might be built up by differential voltages occurring between the ITO and circuitry underlying the gaps between the mirrors.
Because the ITO is switched between V0 and V1, the average voltage on the ITO over time is xc2xd (V0+V1). If we xe2x80x9cplug the gapsxe2x80x9d in the mirrors with metal which is statically held at xc2xd (V0+V1), the residual bias is eliminated. However, such a voltage does not typically exist within the LCD array. Therefore, according to the present invention, the gaps between the mirrors are plugged with equal areas of traces carrying V0 and V1. Therefore, the residual bias (averaged over an area which includes equal areas of V0 and V1) will be zero, thereby essentially eliminating this potential source of residual DC bias.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of modes of carrying out the invention, and the industrial applicability thereof, as described herein and as illustrated in the several figures of the drawing. The objects and advantages listed are not an exhaustive list of all possible advantages of the invention. Moreover, it will be possible to practice the invention even where one or more of the intended objects and/or advantages might be absent or not required in the application.
Further, those skilled in the art will recognize that various embodiments of the present invention may achieve one or more, but not necessarily all, of the above described objects and advantages. Accordingly, the listed advantages are not essential elements of the present invention, and should not be construed as limitations.